The satellite navigation systems GPS (USA), GLONASS (Russia), GALILEO (Europe) etc. are intended for determining the location of users provided with special navigation receivers. The navigation receiver receives and processes the radio signals emitted by satellites located in its direct visual range.
The satellite signals are modulated using a pseudorandom binary code, which is used for measuring the delay in relation to a local reference oscillation. These measurements make it possible to determine pseudoranges which differ from the actual ranges from the satellites as a result of a discrepancy between the time scales on board the satellite and at the user and as a result of measurement errors. If the quantity of satellites is sufficient, by processing the measured pseudoranges it is possible to determine the coordinates of the user and to match the time scales.
The necessity to solve problems which require a high degree of accuracy as well as the attempt to increase the stability and reliability of the measurements have lead to the development of the differential navigation (DN) mode. In the DN mode, the coordinates of the user are determined in relation to a base station (Base), whose coordinates are known with a high degree of accuracy. During the measurements, the base station is usually immovable, while the user can be either immovable or moving. At the Base there is a navigation receiver, which receives and processes the satellite signals. The results of these measurements are transmitted to the user, which also has a navigation receiver.
Using the joint processing of the measurements of the Base and the rover, the user is provided with the possibility of accurately determining its relative coordinates as a result of compensation for the significant proportion of strongly correlated errors. Depending on the problems to be solved in the DN, it is possible to use various modes. In the post-processing mode, the joint processing of the measurements of the base and the rover is performed on the basis of an entry after all of the measurements are complete. In the real-time processing mode, the coordinates of the rover are determined in real time in terms of the arrival of information from the base over the communications line.
The increase in the accuracy of the DN can be achieved if the pseudorange measurements on the basis of codes are supplemented by measurements of the phase of the carrier. By measuring the phase of the carrier of the signal received from the satellite in the base receiver and comparing said phase with the phase of the carrier from the same satellite, which phase of the carrier is measured in the receiver of the rover, it is possible to achieve accuracy in the relative coordinates to several percentage points from the length of the wave of the carrier oscillation.
Implementing the advantages which make it possible to take the measurements on the basis of the phase of the carrier requires that any ambiguity in the phase measurements be resolved. Since the difference in the ranges from the base to the rover are usually significantly greater than the length of the wave, the difference in the phases of the carriers received by the receivers of the base and of the rover can significantly exceed 360 degrees. As a result of this, the measured difference in the phases will differ from the actual difference in the total phases by an integer of cycles (1 cycle corresponds to 360 degrees). This integer of cycles proves to be unknown and needs to be determined on the basis of the results of the measurements together with other unknown values, namely the coordinates of the rover and the discrepancy between the time scales.
Each satellite whose radiation is received by the receiver uses one satellite channel. Dual-frequency receivers receive radiation from satellites in the ranges L1 and L2 and respectively measure the phases at two frequencies f1 and f2, which makes it possible to make corrections to the ionospheric delay and makes it easier to resolve any ambiguity.
In the navigation receiver, signals from different satellites are divided between channels and are processed with the aim of extracting navigation information which is contained in the value of the relative time delays between incoming signals. Radio signals entering the input of the navigation receiver from an antenna are fed to a radio path which is common to signals from different satellites and comprises input and filtering units and frequency converters. Once the satellite signals have passed through the common radio path, said signals are processed separately in digital satellite channels, with there being one channel for each satellite.
The basis of the channel of each satellite comprises two tracking systems, which track the changes in the parameters of the incoming signal. The first tracking system, the “delay-locked loop” (DLL), tracks the changes in the delay of the modulating pseudorandom code in the incoming signal. The second tracking system, the “phase-locked loop” (PLL), tracks the changes in the phase of the carrier in the incoming signal.
In order to form the tracking systems, the signal is converted into digital form and is processed, both using hardware and using software, in a microprocessor, which is part of the receiver. The processing of the signal received includes storing the result of successive multiplication of this signal by the reference carrier and the reference code which are generated in the receiver. The reference carrier corresponds to the carrier of the received signal for this satellite, and the reference code is the respective pseudorandom code (PR code) with which the signal of that satellite was modulated.
The devices performing this multiplication and storing are called “correlators”, and the corresponding process is called the correlation of two signals. The output value of the correlator is the intercorrelation function of the input and reference signals. Each satellite channel in the receiver comprises three parallel correlators.
In the first correlator, the in-phase correlation signal I is calculated. Such a signal is obtained if the first reference carrier, which is in phase with the carrier of the input signal, is used in the correlator. The reference code is a copy of the PR code which modulates the input signal. As a result of the correlation of said signals, the in-phase signal I is formed. Initially, or as a result of errors, the phase of the reference carrier can differ from the phase of the carrier of the input signal, and the delay of the reference code can differ from the delay of the modulating code.
If the phase shift of the first reference carrier is designated by the letter φ and the time shift of the reference code in relation to the input (modulating) code is designated by τ, the signal I shall be determined by the following relationship:I=kUsμRo(τ)cos φ+Iin  (E1)where:
Ro(τ) is the standardized intercorrelation function of the input PR code (having passed through the filter in the common radio path of the receiver) and of the reference code, which is a locally generated copy of the PR code which modulates the satellite signal;
cos φ is the result of the correlation between the carrier of the input signal and the in-phase reference carrier in the event of the presence of a phase shift;
Us is the amplitude of the input signal;
μ=±1 is the information symbol modulating the input signal;
k is the coefficient of proportionality; and
Iin is the interference at the output of the correlator I which is generated as a result of the action of additive interference at the input of the receiver.
The signal I is used for extracting the information symbols and, in the other paths, is used as an auxiliary signal for standardization. In the tracking mode, the values φ and τ are small and Ro(τ) cos φ approaches unity. Furthermore, the in-phase correlation signal I reproduces the sequence of information symbols μ=±1 transmitting messages from on board the navigation satellite to the receiver of the user. The messages contain information on the coordinates of the satellite, the expected characteristics of the conditions for propagation and other data which are used in the coordinate determinations.
The second correlator of the satellite channel calculates the quadrature correlation signal Q. This signal is produced if the second (quadrature) reference carrier whose phase is shifted through π/2 in relation to the first reference carrier, is used and the reference code is identical to the code in the first path. The correlation of said signals (in “correlator Q”) forms the quadrature correlation signal Q, which is determined on the basis of the following relationship:Q=kUsμRo(τ)sin φ+Qin  (E2),where:
Qin is the interference at the output of the correlator Q which is generated by the additive interference at the input of the receiver, and
sin φ is the result of the correlation between the carrier of the input signal and the quadrature reference carrier.
The signal Q is used for generating an error signal in the phase-locked loop (PLL).
The third correlator of the satellite channel calculates the coded correlation signal dI used for generating an error signal with the aim of controlling the Delay-Locked Loop (DLL) for the modulating code. The first reference carrier (which is in phase with the carrier of the input signal) and the reference code, which consists of short strobe pulses corresponding to the times at which the mathematical sign of the elements of the input PR code changes, are used to produce this signal.
The signal dI is determined on the basis of the following relationship:dI=kUsμΔRo(τ)cos φ+dIin  (E3),where:
ΔRo(τ) is the intercorrelation function of the input PR code (once it has passed through the radio path of the receiver) and of the reference code in the form of a sequence of short strobe pulses; and
dIin is the interference at the output of the “correlator dI” which is generated by the additive interference at the input of the receiver.
During operation of the receiver, the values used in the relationships change and, correspondingly, the correlation signals determined on the basis thereof also change. After the calculation, the correlation signals are stored in adders with a dump, in which adders the magnitudes of the correlation signals are stored (once the mathematical signs of the information symbols have been removed):I*=ΣI Q*=ΣQ dI*=ΣdI 
The stored signals I*, Q*, dI* generated in the paths of each channel are used for the joint operation of the tracking systems, namely the phase-locked loop for tracking the carrier frequency and the delay-locked loop for the PR code.
The tracking error signal is determined in a discriminator on the basis of the following relationship:Zd=arctan(Q*/I*).  (E4)The tracking error signal Zd is proportional to the error φ within the range of ±π/2, and then is repeated periodically. The dependence of Zd on φ forms the discriminator characteristic of the PLL. The error signal is sent to a loop filter, which issues a control signal and closes the circuit of the PLL, whilst controlling the phase shift of the numerically controlled oscillator of the channel (NCO). The time shifts in the reference codes in the 1st and 2nd paths are firmly linked in time to the DLL-regulated shift in the reference code of the third path.
The tracking systems PLL and DLL are closed circuits with the task of reducing the tracking errors τ and φ to zero. For this purpose, the error signals are converted into control signals, which change the phase and delay of the oscillators of the reference signals. In real conditions, as a result of the external influences on the tracking systems, the values of the tracking errors are different than zero, but under normal conditions in the tracking mode, the errors are small and fluctuate about the point of stable equilibrium at which the error signal is equal to zero. A system of searching by delay and frequency is used for the tracking systems of receiver channels entering the tracking mode.
In previous technical solutions, the tracking of signals from different navigation satellites has been performed in different satellite receiver channels by independent individual tracking systems: PLL and DLL, and the bands thereof needed to be selected, on the basis of contradictory requirements, taking into account noise errors and dynamic errors. However, the quickest changes in the delay of signals taking effect on these tracking systems are common to all the satellite channels. Movements of the antenna of the user and an offset of the time scale of the receiver of the rover can be added to the common delays.
The U.S. Pat. No. 6,313,789 proposes a method for organizing tracking systems in a navigation receiver, in which signals from all of the satellites being observed are processed jointly in order to track the common changes in the delays. As a result, a series of advantages was obtained owing to the use of the total power of all of the satellites. In addition, the power of the signal from each satellite was used separately for tracking comparatively slow disturbances in the individual loops of the PLL, said disturbances acting solely on the signal from said satellite, independently of the others. The individual loops could be made comparatively narrowband in order to suppress the effect of noise.
The proposed method was implemented by means of creating a vector common loop in the receiver, which consisted of three geometrical common circuits (for three geometrical coordinates to be measured) and a fourth (quartz) common circuit for tracking the fluctuations in the phase of the reference oscillator. All of the circuits in the common loop were identical and the vector common loop was produced so as to be uniform. Apart from the vector common loop in the receiver, individual PLL loops also remained for tracking the phase of the carrier in each channel.
Combining the channels in a uniform vector loop was performed at the output of the PLL discriminators. The output signals of the discriminators from different channels were added and processed in accordance with the method of least squares in order to produce four combined error signals relating to four measured coordinates, and the resultant magnitudes of the error signals were filtered.
For the geometrical circuits, the signals were projected in the directions of vectors of the range from satellites and, when added to the signal of the quartz circuit and the error signals of individual loops, controlled the frequency of the PLL reference oscillators of each channel. In addition, the U.S. Pat. No. 6,313,789 has proposed, for the case of an immovable rover, generating a common phase signal, filtering said signal and using the filtered signal as a correcting phase signal.
In the majority of cases, the receiver of the rover operates in complex conditions with different external influences and interference which are the reason for the measurement errors. It is customary to differentiate between usual (normal) errors which define the accuracy of the measurements and abnormal errors whose values are so great that the corresponding abnormal measurement can represent a substantial hindrance when performing the task addressed by the user. Abnormal errors are occasionally a consequence of inadvertently large inherent noise emissions, but are more often associated with external influences on the receiver.
A particular type of abnormal phase error is a sudden change in phase of the PLL carrier. In the event of an isolated sudden change, the PLL transfers to a new stable point, after which the tracking is continued. As a result, an abnormal error which is a multiple of the integer of half-cycles remains after the sudden change in the measurements of the full phase. In the event of a long duration of the sudden change or a series of sudden changes, an interruption in the PLL tracking is detected and the search system begins to function.
The occurrence of strong re-reflected signals or the exposure to strong radio interference can lead to abnormal errors.
In receivers intended for installation on movable objects, the external dynamic influences play a particular role and the abnormal errors arising in the process are often the cause of interruptions in the tracking. The movement of the rover is accompanied by mechanical disturbances. The frequency of the input signals is offset proportionally to the speed, the operation of the engine causes vibrations of differing frequency, and vibrations, jolts or impacts during movement of the rover cause pulsed accelerations which have an effect on the reception antenna and on the quartz of the local reference oscillator, thereby causing a drift in the intermediate carrier frequency and the measured phase. As a result, PLL errors occur which can lead to an interruption in the tracking.
In order to reduce the probability of interruptions in the tracking, special measures are taken to suppress dynamic disturbances. The method proposed in the U.S. Pat. No. 6,313,789 has provided the solution to the problem of increasing shock resistance by widening the bandwidth of the tracking system. It has been proposed that the structure and parameters of the four circuits of the vector common loop be made identical and be selected from conditions representing a compromise between the value of the fluctuating (noise) errors and the dynamic errors arising during movement.
Since all of the circuits were identical, the vector common loop proved to be uniform. However, under conditions where the receiver was exposed to particularly strong vibrations and jolts (for example for receivers used for control purposes in construction machinery), it often happened that the uniform common loop did not have a sufficient effect.